A construction of a known photothermographic dry silver film or paper product 10 is shown in FIG. 1. This construction can be created by coating a plurality of layers onto a substrate. One of the layers is a phototherrnographic emulsion layer 14 made up of a photosensitized silver soap in a binder resin which can include toners, developers, sensitizers and stabilizers. To improve adhesion of the photothermographic emulsion layer 14 to the substrate, a primer layer 16 can be positioned between them. A topcoat layer 12 can be positioned above the photothermographic emulsion layer 14 and can be made up of a mar-resistant hard resin with toners and slip agents. The substrate 18 can be paper-based substrate or a polymeric film-based substrate. An antihalation layer 20 can be applied to the surface of the substrate 18 opposite the surface on which the primer, photothermographic emulsion, and topcoat layers 16, 14, 12 can be positioned. The compositions of layers 16, 14 and 12 are chosen for product performance reasons, and components comprising adjacent coating layers could be incompatible.
It is desirable to determine how to coat the fluids that form (i.e., the precursors) for the primer, photothermographic, and topcoat layers 16, 14, 12, respectively, using a simultaneous multilayer coating method. Slide coating, as described in U.S. Pat. No. 2,761,419 (Mercier et al., 1956) and elsewhere (see E. D. Cohen and E. B. Gutoff, Modern Coating and Drying Technology, VCH Publishers, 1992), is a method for multilayer coating, i.e., it involves coating a plurality of fluid layers onto a substrate. The different fluids comprising the multiple layer precursors flow out of multiple slots that open out onto an inclined plane. The fluids flow down the plane, across the coating gap and onto an upward moving substrate. It is claimed that the fluids do not mix on the plane, across the coating gap, or on the web, so that the final coating is composed of distinct superposed layers. A number of developments have been reported in this area regarding the use of slot steps, chamfers, and have been described in literature (see E. D. Cohen and E. B. Gutoff, op. cit.).
The application of multilayer slide coating as described in the above references to the coating of a product such as is described in FIG. 1, that involves coating layers comprising incompatible solutes in miscible solvents, can lead to a problem of "strikethrough" that is described herewith. Incompatible solutes are solutes that do not mix in some or all concentration ranges, whereas miscible solvents are solvents that mix in any proportion.
Occasionally during coating, a disturbance causes one of the coating layers above the bottom-most coating layer to penetrate through the bottom-most coating layer to the slide surface. When the solute of the coating layer(s) above the bottom-most coating layer is sufficiently incompatible with the solute of the bottom-most layer, the penetrating coating layer attaches to slide surface 53 and is not quickly self-cleaned by the bottom-most coating layer. This phenomenon is referred to as strikethrough. (The term "self-clean" means the process which occurs when the flow of the bottom-most coating layer (or the bottom-most coating layer and one or more adjacent coating fluid layers) cleans off the penetrant coating fluid layer that sticks to the slide surface.)
When strikethrough occurs, the flow of the coating fluid down the slide surface 53 is disturbed which can lead to streaking defects in the coated product. Streaking defects can, in turn, reduce product quality to the point where the final product is outside specifications and cannot be used.
Another problem encountered during multilayer slide coating of product constructions involving different solvents in different layers is that the interdiffusion of solvents between these layers can cause phase separation of one or more solutes within one or more layers.
This phase separation can result in the inability to coat such a construction using a multi-layer coating technique due to formation of defects such as streaks or fish-eyes, or due to a disruption of flow and the intermixing of separate fluid layers.
Traditional slide coating, as described in U.S. Pat. No. 2,761,419 (Mercier et al., 1956), is restricted to coating solutions that are relatively low in viscosity. The use of a "carrier layer" in slide coating was first described by U.S. Pat. No. 4,001,024 (Dittman and Rozzi, 1977), where the authors claimed an improvement over a previously-described method of slide coating "by coating the lowermost layer as a thin layer formed from a low viscosity composition and coating the layer above the lowermost layer as a thicker layer of higher viscosity." Furthermore, the authors state that due to the vortical action of the coating bead that is confined within the two bottom layers, intermixing occurs between the two bottom layers, and, therefore, the coating compositions of these two layers must be chosen such that the interlayer mixing is not harmful to the product. However, this patent does not address strikethrough or phase separation.
U.S. Pat. No. 4,113,903 (Choinski, 1978) teaches that a low viscosity carrier layer tends to be unstable "in the bridge between the coater lip and the web in the bead formed with a bead coater" and can limit the web speed at which the method can be applied. To overcome this problem, Choinski suggests use of a non-Newtonian pseudoplastic liquid as the carrier, such that it has a high viscosity on the slide and in the bead where the shear rate is low, and a low viscosity near the dynamic contact line where the shear rate is high. In U.S. Pat. No. 4,525,392 (Ishizaki and Fuchigami, 1985), it is further specified that the non-Newtonian (or shear thinning) carrier layer viscosity should be within 10 cp of the next layer at low shear rates, but lower at high shear rates. However, these patents do not address strikethrough or phase separation.
Interlayer mixing between the bottom two layers "caused by a whirl formation in the meniscus" is cited as a limitation of the above patents, and a method of overcoming this interlayer mixing by adjustment of coating gap is described in U.S. Pat. No. 4,572,849 (Koepke et al., 1986). This method also employs a low viscosity accelerating layer as the lowermost layer over which other higher viscosity layers can be arranged. A slightly different layer arrangement is also described where a low viscosity spreading layer is used as the uppermost layer in addition to the lowermost low viscosity accelerating layer. The same arrangement is used for curtain coating in related patent U.S. Pat. No. 4,569,863 (Koepke et al., 1986). However, neither patent addresses the problem of strikethrough or phase separation that occurs on the slide surface.
U.S. Pat. No. 4,863,765 (Ishizuka, 1988) teaches that using a thin layer of distilled water as carrier allows high coating speeds and also eliminates mixing between the two lowermost layers. In related patents U.S. Pat. No. 4,976,999 and U.S. Pat. No. 4,977,852 (Ishizuka, 1990a and 1990b), the carrier slide construction with water as carrier (as described in U.S. Pat. No. 4,863,765) is used, and it is noted that streaking is reduced by using smaller slot heights for the carrier layer and that bead edges are stabilized by extending the width of the carrier layer beyond the width of the other layers coated above the carrier. This patent also does not address strikethrough or phase separation.
In summary, U.S. Pat. Nos. 4,001,024, 4,113,903, and 4,525,392 require that the composition of the two bottom layers be adjusted such that interlayer mixing between these layers in the coating bead not lead to defects in the product. U.S. Pat. No. 4,572,849 (and related U.S. Pat. No. 4,569,863), while not restricting layer composition, restricts, the coating gap to the range 100 .mu.m-400.mu.m. Likewise, U.S. Pat. Nos. 4,863,765, 4,976,999 and 4,977,852, while not specifically requiring a composition adjustment, are restricted to aqueous solutions by use of distilled water as carrier. However, the problem of strikethrough that occurs with a product construction as shown in FIG. 1 is not addressed by these patents. In other words, the prior art as described in the above patents does not disclose the necessary criteria that will allow strikethrough-free manufacture of a product such as a photothermographic element that is illustrated in FIG. 1. Furthermore, these patents do not address the problem of phase separation that can prevent the use of a multi-layer coating technique in the manufacture of a product, such as the product illustrated in FIG. 1.
It would be desirable to simultaneously apply such incompatible solutes in miscible solvents using multilayer coating techniques such as slide coating without occurrence of strikethrough or phase separation. It would also be desirable to continuously coat such compositions at wide coating gaps (greater than 400 .mu.m) to allow for coating over splices in the substrate without interruption in order to maximize productivity. Moreover, it would be desirable to apply such layers from either organic solvent or aqueous medium, as required by product composition.
Still further, it would be desirable to reduce the waste of coating fluid(s) that results when it becomes necessary to interrupt the coating process. When slide coating is begun, a uniform, streak-free flow of each of the fluid layers on the slide surface is established. This is often a careful, tedious, and time-consuming process. Only after streak-free, stable, uniform fluid flows are established is the coating die moved toward the moving web to form a coating bead and thus transfer the coating to the web. When coating must be interrupted during the normal course of coating operations, the coating die is retracted from the web.
Often when this is done, the flow of coating fluids is continued to insure that pumping and streak-free, stable, uniform fluid flows are maintained. The coating fluid(s) are collected by a vacuum box trough or drain trough and drained to a scrap receptacle. This has the disadvantage of wasting coating fluid(s).
Alternatively, to minimize waste of coating fluid(s) during prolonged pauses in coating, the flow of coating fluid(s) is often completely stopped and some covering such as tape is placed over the coating die slots to reduce drying. Unfortunately, this leads to contamination of the slide and slots by adhesive, particles, fibers, etc., and is only marginally effective in preventing dry-out and/or coagulation in the slots. When coating is resumed, the tedious process of streak elimination must be repeated, and streak-free, stable, uniform fluid flows must be reestablished. This can, again, result in waste of coating fluid(s) and loss of production time.
Yet another alternative is to reduce rather than completely stop the flow of coating fluid(s). When this method is used with volatile organic solvent based coatings, undesirable dry-out and/or coagulation of the coating fluid(s) on the slide surface and in the slide slots still occurs due to the rapid evaporation of the volatile organic solvent. Again, when coating is resumed, streak elimination must be repeated, and stable fluid flows must be reestablished.
It would be desirable to find a method that avoids either the need for continuous flow of the coating fluid, or streaks, dryout, etc., that can result during necessary interruptions to the coating process. This desire and other desires noted herein extend beyond the process of making photothermographic, thermographic, photographic, and data storage materials (such as magnetic storage media) to the preparation of other coated materials whose production involves similar problems.